System and method for producing pressurized hydrogen from a solid oxide electrolyser connected to an electrochemical hydrogen compressor

Abstract

The invention relates to a system and method for producing pressurized hydrogen from a solid oxide electrolyser connected to an electrochemical hydrogen compressor The system comprises a solid oxide electrolyser (SOEC) (1), which is configured to generate hydrogen; an electrochemical hydrogen compressor (EHC) (2), which is configured to pressurize said hydrogen generated by said SOEC; and a first recovery circuit, which is configured to recover water exiting the cathode (2c) of the EHC (2) by providing a return path through the EHC (2) to the cathode (1C) of the SOEC (1) for consumption. An optional second recovery circuit is configured to recover heat from at least one output flow (4, 5) of the SOEC (1) to a heat exchanger (15), which is configured to heat said return path (4, 18) at the cathode (1C) of said solid oxide electrolyser (1).

Description

[0001]SYSTEM AND METHOD FOR PRODUCING PRESSURIZED HYDROGEN FROM A SOLID OXIDE ELECTROLYSER CONNECTED TO AN ELECTROCHEMICAL HYDROGEN COMPRESSOR. FIELD The invention relates to the field of electrochemistry, more particular to electrochemical cellssuch as solid oxide electrolysers, with the aim of producing hydrogen, and to electrochemicalcompressors, with the aim of compressing hydrogen.BACKGROUND Water electrolysis is used to electrochemically reduce water and decompose it into hydrogen and oxygen. By using fuel cell technology in reverse mode, water electrolysis is an environmentally friendly way of producing hydrogen energy. Solid oxide cells can be used as solid oxide electrolyzer cells (SOEC) to produce hydrogenand oxygen from water under normal pressure and elevated temperatures. In SOEC cells,oxygen ions are transported from the cathode to the anode by the chemical gradient of partial pressure of oxygen across the electrolyte, and by a positive potential applied to theanode of the cell. SOEC electrolysis systems have excellent efficiency, but this may becompromised by production costs, if not enough waste heat or affordable electricity isavailable to power the steam generators presently needed to evaporate water into steam.Transportation of hydrogen in pressurized or in liquid form is viable, if the processing of thehydrogen can be made economical. Pressurization of hydrogen is achieved, with differentmechanical or non-mechanical compressors, of which electrochemical hydrogencompressors (EHC) belong to the latter category. These isothermally operated compressorsare currently based on polymer electrolyte membranes (PEM) that operate from roomtemperature up to about 100 °C. PEM-based electrochemical compressors operate atefficiencies that are comparable or better to the best mechanical compressors. They arereliable and do not contaminate hydrogen with oil. However, operating polymer membranesat low temperatures require a pure hydrogen feed stream.SOEC and EHC devices as described herein in fact consist of stacks of their respective cells,but for clarity and in this context, the concept is not used.To maintain isothermal compression, a great deal of heat must be removed as typically mostof the work put into the EHC system leaves as heat. Removing this heat effectively from anEHC can be challenging.Thus, a more energy-efficient process of operating solid oxide electrolysers to producehydrogen and to compress the hydrogen in an electrochemical hydrogen compressor, issought after. SUMMARYThe present invention aims at a system and a method for providing SOEC- generated andpressurized hydrogen with a process that significantly improves the overall energy efficiency of the process compared to known prior art. Protons travelling from an anode to a cathode tend to drag water molecules with them. This flux is called electro-osmotic drag (EOD) and results in anode water depletion and membrane resistance increment. Accordingly, fuel cell membranes must be kept humid by some means.It is herein proposed a method and a system for producing pressurized hydrogen by a solidoxide electrolyser (SOEC) and an electrochemical hydrogen compressor (EHC), which are in fluid connection with each other. According to a first aspect of the present disclosure there is provided a system for producing pressurized hydrogen. The system includes a solid oxide electrolyser (SOEC), which is configured to generate hydrogen, an electrochemical hydrogen compressor (EHC), which is configured to pressurize said hydrogen generated by said SOEC, and a first recovery circuit, which is configured to recover water exiting the cathode of the EHC by providing a returnpath for said recovered water through the EHC to the cathode of the SOEC for consumption.According to a second aspect of the present disclosure there is provided a method for producing pressurized hydrogen. The method involves generating hydrogen in a solid oxide electrolyser (SOEC), pressurizing the hydrogen generated in said SOEC in an electrochemical hydrogen compressor (EHC), providing a first recovery circuit for recovering water from the cathode of the EHC and returning it through the EHC anode to the cathode ofthe SOEC, and providing said recovered water to the SOEC for its consumption. Certain embodiments may include one or more than one additional feature from the following itemized list: ^the EHC is configured to vaporize at least part of the water received at the cathode atthe membrane of the said ECH,^ the anode is integrated to the membrane to form a membrane electrode assembly,^ the membrane is configured to diffuse water, which is received at the cathode, to theanode for vaporization, ^the recovery circuit features a water inlet,^ the water inlet is configured to feed water in liquid state to the cathode of the EHC,^ said first recovery circuit comprises a water recovery circuit,^ the water recovery circuit has a separator for separating water from the cathode ofthe EHC from hydrogen, ^the water recovery circuit has a tank for said separated water,^ the water recovery circuit has a pump for pumping water back to the EHC cathode,^ the pump is connected to said tank for replenishment of water to the tank from anexternal source, ^the system further comprises a heat recovery circuit,^ the heat recovery circuit is configured to recover heat from at least one output flow ofthe SOEC to a heat exchanger, ^the heat exchanger is configured to heat said return path at the cathode of said solidoxide electrolyser, ^the system comprises a sweep gas system, which is configured to feed air or othergas to the anode of the SOEC,^ the system comprises a sweep gas heat exchanger, which is configured to pre-heatthe sweep gas fed to the anode of the SOEC by the sweep gas system, ^said heat recovering circuit comprises a first heat exchanger,^ the first heat exchanger is at an oxygen output flow from the SOEC,^ heat is recovered at the SOEC oxygen output flow and fed to a second heatexchanger to heat the feed from the anode of the EHC to the cathode of the SOEC, ^said heat recovering circuit comprises a third heat exchanger,^ the third heat exchanger is at the hydrogen output flow from the SOEC,^ heat is recovered at the SOEC hydrogen output flow and fed to said second heatexchanger to heat the feed from the anode of the EHC to the cathode of the SOEC, ^A system according to any of claims 1 - 6, wherein the EHC (2) is encapsulated in apressure vessel (12) to maintain a pressure at least across the EHC (2) and said water recovery circuit (6, 8, 9).^ the membrane of said electrochemical hydrogen compressor is a polymer electrolytemembrane (PEM), ^the membrane of said electrochemical hydrogen compressor is configured tovaporize at least part of the water received at the cathode, ^in said first recovery circuit, water from the cathode of the EHC is separated fromhydrogen and fed to a tank to be pumped back to the EHC cathode, ^providing at least one second recovery circuit for recovering heat in at least oneoutput flow from the SOEC, ^using said recovered heat to heat the feed from the anode of the EHC to the cathodeof the SOEC, ^recovering heat in said second recovery circuit from an oxygen output flow from theSOEC in a first heat exchanger, ^feeding said recovered heat to a second heat exchanger to heat the feed from theanode of the EHC to the cathode of the SOEC, ^recovering heat in said second recovery circuit from the hydrogen output flow fromthe SOEC in a third heat exchanger, ^feeding said recovered heat to said second heat exchanger to heat the feed from theanode of the EHC to the cathode of the SOEC, ^vaporizing at least part of the water received at the cathode at the membrane of thesaid electrochemical hydrogen compressor.The present disclosure presents a first finding that, when liquid water is circulated outside thecathode, water will be back-diffused to the anode, where large amount of water willevaporate, if the EHC anode is operated isothermally at 100 ^C and at atmospheric pressure(1 bar). The waste heat generated in the EHC is mainly due to the ohmic resistance andcharge transfer resistance when hydrogen molecules are split by the electric current and theprotons are pumped to the opposite side of the membrane. This waste heat is now used toevaporate water on the anode, which when fed into the SOEC, will be consumed forproduction of hydrogen and oxygen.This water feed has the additional advantage that the hydrogen-water mix entering the EHCfrom the SOEC is virtually oxygen-free. This is important, because at 100 ^C, already smallamounts of oxygen may degrade the EHC membrane and its catalysts.A second finding is that waste heat from the SOEC, which is inherently working at highertemperatures (^ 600 - 800 ^C) than the EHC, also can be recovered and used to heat thewater vapor feed from the EHC to the SOEC.If combined, these sources of waste heat may completely eliminate the need for electricallyheated steam generators, excluding during a system startup phase.In this way, feed loops of water and / or heat energy may be established between the SOECand the EHC, thus improving the energy efficiency of the overall process to producepressurized hydrogen. The energy efficiency is calculated to increase with 10-15 %, optimally20 % with low current density, compared to a situation where only electrically heated steamgenerators are used.In the context of this disclosure, the terms “solid oxide electrolyser” and “solid oxide electrolyser cell” are used as synonyms The specific scope of the inventive system and method is set out in the appended claims. BRIEF DESCRIPTIONS OF THE DRAWINGS Fig.1 shows connected SOEC and EHC devices capable of supporting at least someembodiments of the inventive system;Fig.2 shows connected SOEC and EHC devices capable of supporting at least some further embodiments of the inventive system; Fig.3 is a flowchart schematically showing the water and hydrogen flow in the SOEC andEHC devices according to at least some embodiments of the inventive system.DETAILED DESCRIPTION OF EMBODIMENTSIn Fig. 1, a system of an SOEC interconnected to an EHC is shown. Interconnected is here aterm used to describe the mutual fluid connections between the SOEC and the EHC, as will be described in detail below. An SOEC 1 device with an anode 1A and a cathode 1C is shown. Water is consumed, as thefollowing reactions take place in the SOEC:Cathode: H2O + 2 e− → H2 + ½ O2− [1]Anode: 2 O2− → O2 + 4 e− [2]Oxygen is released at 5 from the SOEC 1 to atmosphere, or is utilised otherwise.An almost oxygen-free mixture of hydrogen and water vapour are conducted with a fluid connection 3 to the anode 2A of an EHC 2, where hydrogen is electrochemically pumped to the cathode 2C of the EHC 2. The reactions in an electrochemical hydrogen pump EHC are:Anode: H2 -> 2 H+ + 2 e- [3]Cathode: 2 H+ + 2 e- -> H2 [4]As a result, the EHC is pumping low pressure hydrogen from the anode side 2A to the highpressure (cathode) side 2C of the EHC. While the operating temperature of the EHC anode2A in an isothermal compression mode is here about 100 ^C, the EHC is operating with liquidwater circulation on the cathode 2C.More specifically, there is provided a first recovery circuit recovering water collected from thecathode 2C of the EHC 2 by returning it through the EHC to the cathode 1C of the SOEC,where it is used to produce oxygen and hydrogen. This is achieved as follows:The EHC 2 is operated with a liquid water circulation mode on the cathode. In this mode,hydrogen produced on the cathode side of the EHC 2 is mixed with liquid water and the mixis circulated in the cathode 2C. This circulation maintains and stabilizes the temperature inthe cathode 2C, and also facilitates the separation between gaseous hydrogen and liquidwater. When this hydrogen-water mixture exits the cathode 2C, water is separated fromhydrogen with any suitable industry-standard gas-liquid separation process, resulting in liquidwater flowing out from a fluid connection at 6 and the humid hydrogen at 7. The liquid water flows to a tank 8, from where it is pumped back to the EHC cathode 2C by a water pump 9.Water is in liquid form in the pressurized cathode 2C and transferred 2B through themembrane 2M to the anode 2A, where it is vaporized due to the low pressure (1 bar) and theexcess heat in the EHC. The excess heat is mostly due to resistive losses in EHCcomponents. The humidity from the cathode 2C is transferred through the EHC membrane2M to the anode 2A due to back-diffusion. This is caused by the difference in waterconcentration between the anode 2A and the cathode 2C. The anode is dryer, why water istravelling from the cathode 2C to the anode 2A. This water flux is significant in volume.Therefore, there is a frequent and controlled refilling of the water tank 8, which is provided forby a water pump 11. The anode 2A may be integrated to the membrane 2M to form a membrane electrodeassembly. The EHC 2 thus vaporizes at least part of the water, which is received at thecathode 2C of the membrane 2M and diffused to the anode 2A, where the water is vaporized. Majority of the vaporizing will occur at the anode 2A. As the cathode side of theEHC 2 is under great pressure, e.g. between 5 and 30 bar, minor or insignificant vaporizationmay occur on the cathode 2C. It is preferable to keep a pressure differential of about 10 to 30 bar across the membrane 2M. The EHC water feed 6, 8, 9 of Fig.2 may be modified in various ways. Instead of a tank 8,the water source may be an open vessel or an active water line. Also the pump 9 is optional,as the water may be pressurized in another way, e.g. by gravity or by having latent pressureprevalent in active water lines. Indeed the recovery circuit features a water inlet 21, which isconfigured to feed water in liquid state to the cathode 2C of the EHC 2. Water vapor and any remaining part of hydrogen is pumped from the EHC anode 2A to the cathode 1C of the SOEC 1 using a circulation pump 4, which completes the water recoverycircuit. The pump 4 is preferably located in the coolest part of the circuit, i.e. at the EHCanode 2A outlet, where the temperature is about 100 ^C.From a process perspective the SOEC product, hydrogen, is supplied to the EHC anode 2A,where most of the hydrogen is pumped to the EHC cathode 2C and out of the system at exit7. The rest of the hydrogen is humidified in the EHC by the water recycling circuit 6, 8 and 9,passed 2B from the cathode 2C to the anode 2A, where humidity rich exit gas of EHC anode2A is fed back to the SOEC 1 via a pipe 18, where the humidity is consumed in reactions [1]and [2].According to an embodiment, there may further be provided at least one second recoveringcircuit, wherein heat in at least one output flow from the SOEC 1 is recovered to at leastassist in pre-heating of the water vapor and hydrogen returned to the cathode 1C of theSOEC. This is also shown in Fig.1, where first, second and third heat exchangers (HEX) 13,14 and 15 are shown. The first HEX 13 may be used for the recovery of energy in hot oxygen at the oxygen outlet 5. The third HEX 14 may be used for cooling of the hydrogen-watermixture at 3 before it is fed to the EHC anode 2A. The second HEX 15 may be used to pre-heat the water-hydrogen mixture before it is fed to the SOEC cathode 1C. The specific heat recovery methods and devices used may vary, and are subject to design choices apparent to one skilled in the art. The heat recovered from at least the first and / or the third heatexchangers 13 and 14 may be fed (by appropriate fluids and piping, see Fig.2) to the secondheat exchanger 15 to heat the water-hydrogen mixture and vaporize it and / or keep it ingaseous form, when it is passing through the second heat exchanger 15. A typicaltemperature target for the mixture in the inlet pipe 18 would be 400 ^C.In this arrangement, the energy for evaporating the water and the heating of the water-hydrogen mixture may come from the combined waste heat of the EHC 2 and the SOEC 1.The dimensioning and operation point of the EHC can be chosen so that most of the wasteheat can be utilised for water evaporation, and additional electricity needed for the waterevaporation at the cathode 1C of the SOEC can be minimised. If the waste heat generatedby the SOEC and EHC is not sufficient to evaporate all water needed in the SOEC 1, e.g.electrical heating or heat from other sources may be used to fulfil the evaporation task. Sucha situation may arise e.g. in low power modes, when waste heat obtainable from the SOEC 1and EHC may be reduced. EHC produces enough heat for evaporation only when it is operating on high current densities. The, External electrical or otherwise heating may beprovided e.g. at the EHC cathode 2C water circuit input, after pump 9.According to some embodiments, like the one presented in Fig.1, the EHC 2 is operatingand the pressurized hydrogen is released at inside a pressure vessel 12. The pressurevessel 12 is beneficial for the purpose of providing the pressure needed for the high-pressure cathode side of the EHC. By providing this pressure with a dedicated pressure vessel 12, the structure of the EHC stack may be simplified, as there is no pressure difference betweenEHC cathode and interior of the pressure vessel. The pressurized hydrogen as well as somewater vapour is lead out from the pressure vessel via a valve 10.According to some embodiments, as shown in Fig. 2, the EHC pressure vessel 12 may beomitted. The SOEC-EHC setup is in Fig. 2 quite similar to the one shown in Fig.1. The waterfeed components 6, 8 and 9 in the water recovery circuit preferably operate under the samepressure as EHC cathode, in order to facilitate the gas-liquid separation between hydrogenand water. The pressurized hydrogen outlet from the whole system is now at pipe 7, whichmay be provided with a suitable valve 10 and other gas transfer means. Also in Fig.2, the interconnection between HEX vessels 13, 14 and 15 has been drawn to better visualize the heat recovery cycle. In addition to and clarification of what has beenexplained above, heat recovered from the first heat exchanger 13 is transported by a fluid medium through pipe 16 to the second heat exchanger 15. Similarly, heat from the third heatexchanger 14 is transported by a fluid medium through pipe 17 to the second heat exchanger15. In the second heat exchanger 15 the received heat is consumed to heat the water-hydrogen mixture from the EHC anode 2A and pass it on by pipe 18 to the cathode 1C of thesolid oxide electrolyser cell 1. An optional heat exchanger 19 is shown connected to the pipe 16, to pre-heat a sweep gasthat is blowed into the anode 1A of the SOEC by a fan 20. Pre-heated sweep gas, normallyair, is usually fed to the anodes of commercially available SOEC’s.Fig.3 is a flowchart schematically showing the water and hydrogen flow in the SOEC andEHC devices according to certain embodiments. The aim is to illustrate the inventive firstrecovery circuit in a streamlined fashion.The SOEC anode 1A and cathode 1C as well as the EHC anode 2A and cathode 2C are arranged in a top-down flowchart sequence. On the left side of the flowchart, a downward-pointing arrow “H2O+H2 flow” shows the flow of produced hydrogen and unreacted watervapor in the SOEC hydrogen producing and the EHC hydrogen compression processes, ending in the H2 exit 7 and the water tank 8. The state (phase) of water flowing between theanodes and cathodes 1A,1C,2A,2C is shown by the symbols for water vapor (g) and liquidwater (l), respectively. The state of water in each device is also shown in the upper rightcorner of the device by the same symbols, and the corresponding reactions [1] – [4] areshown on the left side of each device. The typical operating temperatures in each device arealso shown at each component.On the right side of the flowchart, an upward-pointing arrow “H2O return feed” shows thewater and remains of hydrogen flow in the reverse direction in a similar fashion as describedabove for the water and hydrogen flow. Water, in at least part recovered from the downward“H2O+H2 flow” and routed to the tank 8, is pumped to the cathode 2C of the EHC. In thedownward “H2O+H2 flow” only hydrogen (H2) will pass from the anode 2A to the cathode 2Cof the EHC, while water in vapor (g) state remains in EHC anode. Replenishment water (l) ispumped to the tank 8 with pump 11, which is shown in Figure 2 but omitted from Figure 3 forthe sake of simplicity. From the cathode 2C water is transferred to the anode 2A by to back-diffusion and vaporized, as has been described above. The water vapor-hydrogen mix isthen transferred to the SOEC device, as also described above.It is clear to one skilled in the art that various embodiments of the invention are not limited tothe examples described, but may vary within the scope of the appended claims.

Claims

CLAIMS 1. A system for producing pressurized hydrogen, the system comprising:^ a solid oxide electrolyser (SOEC) (1), which is configured to generate hydrogen;^ an electrochemical hydrogen compressor (EHC) (2), which is configured topressurize said hydrogen generated by said SOEC; and^ a first recovery circuit, which is configured to recover water exiting the cathode(2C) of the EHC (2) by providing a return path for said recovered water through theEHC (2) to the cathode (1C) of the SOEC (1) for consumption.

2. The system according to claim 1, wherein the EHC (2) is configured to vaporize atleast part of the water received at the cathode (2C) at the membrane (2M) of the saidECH (2).

3. The system according to claim 1 or 2, wherein the anode (2A) is integrated to themembrane (2M) to form a membrane electrode assembly, which membrane (2M) isconfigured to diffuse water, which is received at the cathode (2C), to the anode (2A)for vaporization.

4. The system according to any one of the preceding claims, wherein the recoverycircuit features a water inlet (21), which is configured to feed water in liquid state tothe cathode (2C) of the EHC (2).

5. The system according to any one of the preceding claims, wherein said first recoverycircuit comprises a water recovery circuit having a separator (6, 7) separating waterfrom the cathode (2C) of the EHC from hydrogen, a tank (8) for said separated water,and a pump (9) for pumping water back to the EHC cathode (2C).

6. The system according to claim 5, wherein a pump (11) is connected to said tank (8)for replenishment of water to the tank from an external source.

7. The system according to any one of the preceding claims, wherein the system furthercomprises at least one second recovery circuit, which is configured to recover heat from at least one output flow (3, 5) of the SOEC (1) to a heat exchanger (15), which is configured to heat said return path (4, 18) at the cathode (1C) of said solid oxide electrolyser (1).

8. The system according to claim 7, wherein said second recovering circuit comprises afirst heat exchanger (13) at an oxygen output flow (5) from the SOEC (1), where heatis recovered and fed to a second heat exchanger (15) to heat the feed from the anode (2A) of the EHC to the cathode (1C) of the SOEC (1).

9. The system according to claim 7 or 8, wherein said second recovering circuitcomprises a third heat exchanger (14) at the hydrogen output flow (3) from the SOEC(1), where heat is recovered and fed to said second heat exchanger (15) to heat thefeed from the anode (2A) of the EHC to the cathode (1C) of the SOEC (1).

10. The system according to any one of the preceding claims, wherein the EHC (2) isencapsulated in a pressure vessel (12) to maintain a pressure at least across the EHC (2) and said water recovery circuit (6, 8, 9).

11. The system according to any one of the preceding claims, wherein the membrane(2M) of said electrochemical hydrogen compressor (2) is a polymer electrolyte membrane (PEM).

12. The system according any one of the preceding claims, wherein the membrane (2M)of said electrochemical hydrogen compressor (2) is configured to vaporize at least part of the water received at the cathode (2C).

13. A method for producing pressurized hydrogen, the method comprising the steps of:generating hydrogen in a solid oxide electrolyser (SOEC); ^pressurizing the hydrogen generated in said SOEC (1) in an electrochemicalhydrogen compressor (EHC); ^providing a first recovery circuit for recovering water from the cathode (2C) of theEHC and returning it through the EHC anode (2A) to the cathode (1C) of the SOEC; and ^providing said recovered water to the SOEC for its consumption.

14. The method according to claim 13, wherein in said first recovery circuit, water fromthe cathode (2C) of the EHC is separated (6, 7) from hydrogen and fed to a tank (8) to be pumped (9) back to the EHC cathode (2C).

15. The method for producing pressurized hydrogen according to claim 13 or 14, themethod comprising the further steps of: ^providing a at least one second recovery circuit for recovering heat in at least oneoutput flow (3, 5) from the SOEC (1); and^ using said recovered heat to heat the feed from the anode (2A) of the EHC to thecathode (1C) of the SOEC (1).

16. The method according to claim 15, wherein the method comprises the further stepsof: ^in said second recovery circuit, recovering heat from an oxygen output flow (5)from the SOEC (1) in a first heat exchanger (13); and^ feeding said recovered heat to a second heat exchanger (15), to heat the feedfrom the anode (2A) of the EHC to the cathode (1C) of the SOEC (1).

17. The method according to claim 15 or 16, wherein the method comprises the furthersteps of: ^in said second recovery circuit, recovering heat from the hydrogen output flow (3)from the SOEC (1) in a third heat exchanger (14);^ feeding said recovered heat to said second heat exchanger (15), to heat the feedfrom the anode (2A) of the EHC to the cathode (1C) of the SOEC (1).

18. The method according to any one of the preceding claims 13 to 17, comprisingvaporizing at least part of the water received at the cathode (2C) at the membrane

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